Agrobacterium harboring a Ti or a Ri plasmid can efficiently transfer a portion of these plasmids, the T-DNA, into plant cells at the site of a wound. Transfer of the T-DNA into the plant cell is induced by signal compounds present at the site of a plant wound and requires T-DNA border sequences at both ends of the T-DNA and trans-acting virulence gene products (vir) encoded by the Ti or Ri plasmid. The transferred T-DNA is then targeted to the nucleus and integrated into the plant genome. A tumor, crown gall, forms at the site of inoculation in most dicotyledonous plants.
Tumor formation results from the expression of T-DNA oncogenes encoding the growth factors auxin and cytokinin which promote plant cell proliferation. In order for these oncogenes to be expressed, the T-DNA must first integrate into the genome of the plant. Tumor formation is limited to dicotyledonous plants because the T-DNA, while transferred into the cells of monocots, is either not normally integrated into the genome of monocotyledonous plants or is integrated and silenced.
By inserting a viral genome into the T-DNA, Agrobacterium can be used to mediate the viral infection of plants. Following transfer of the T-DNA to the plant cell, excision of the viral genome from the T-DNA (mobilization) is required for successful viral infection. This Agrobacterium-mediated method for introducing a virus into a plant host is known as agroinfection (for a review, see Grimsley, “Agroinfection” pp. 325-342, in Methods in Molecular Biology, vol 44: Agrobacterium Protocols, ed. Gartland and Davey, Humana Press, Inc., Totowa, N.J.; and Grimsley (1990) Physiol. Plant. 79:147-153, the contents of which are incorporated herein by reference). Upon entry into the plant cell nucleus, a unit length circular viral genome that is capable of initiating systemic infection is mobilized from the T-DNA. Integration of the T-DNA into the plant genome is not required for this event. Two non-exclusive mechanisms, intramolecular homologous recombination and replicative release, have been proposed for this release of circular viral genomes or replication intermediates from T-DNA. The replicative release of viral genomes by the rolling-circle replication mechanism has been demonstrated for the mobilization of geminivirus genomes from T-DNA (Stenger et al. (1991) Proc. Natl. Acad. Sci. 88:8029-8033). Release of geminivirus via homologous recombination between tandemly repeated genomes has also been demonstrated (Lazarowitz et al. (1989) EMBO J. 8:1023-1032).
Release by either of the above mechanisms requires the presence of tandem directly repeated copies of the viral replicon in the T-DNA. A circular viral replicon may be excised from the T-DNA by intramolecular homologous recombination between the repeated genomes. For replicative release, two origin of replication sequences must be present to initiate and complete the replication process. Both mechanisms of mobilization are complex biochemical processes that may be attenuated by a number of factors which in turn affect the efficiency of viral excision. Furthermore, tandem dimers of viral DNA are often difficult to construct and are unstable in recombination proficient host cells.
Agroinfection has been reported in a number of publications as a successful method for inducing systemic viral infections in plant cells, including monocotyledonous plants such as maize (Heath et al. (1997) Mol. Plant-Microbe Interact. 10:221-227, Grimsley et al. (1989) Mol. Gen. Genet. 217:309-316, the contents of which are incorporated herein by reference). In many instances, particularly when naked viral nucleic acid is non-infectious, agroinfection is the only way of transforming a plant with cloned viral DNA. Even where naked viral nucleic acid is infectious, agroinfection is frequently used because it is relatively efficient and does not require the production of large amounts of plasmid or viral DNA. Agroinfection has been used to study viral replication and recombination, in the investigation of viral gene functions, for the production of autonomously replicating viral vectors, for transient expression of genes inserted into T-DNA, for integration of DNA into a plant genome, for production of virus resistant plants, for the study of transposable elements and for the determination of tissue-specific susceptibility to T-DNA transfer.
The development of plant virus gene vectors for expression of foreign genes in plants provides a means to provide high levels of gene expression within a short time. The benefits of virus-based transient RNA and DNA replicons include rapid and convenient engineering coupled with flexibility for expeditious application in various plant species. In this manner, autonomously replicating viruses offer numerous advantages for use as vehicles for transient expression of foreign genes, including their characteristic high levels of multiplication and concomitant levels of transient gene expression. Accordingly, it would be beneficial to provide methods that facilitate the construction of vectors for agroinfection, provide flexibility in designing viral vectors for genetic transformation of plant cells and increase the efficiency of mobilization of viral replicons from T-DNA and the copy number of a DNA sequence of interest associated with the viral replicon.
The present invention accomplishes these objectives by providing methods and compositions for the mobilization of viral replicon from T-DNA via site-specific recombination systems. Site-specific recombination-mediated excision of DNA fragments from chromosomal or extrachromosomal DNA molecules has been described for a number of site-specific recombination systems and plant species. See Russell et al. (1992) Mol. Gen. Genet. 234:49-59; Lyznik et al. (1996) Nucleic Acids Res. 24:3784-3789; and Dale et al. (1991) Proc. Natl. Acad. Sci. 88:10558-10562 the contents of which are incorporated by reference. However, mobilization of viral vectors from T-DNA via site-specific recombination has not previously been applied to agro-mediated transformation.